Sensors, signal conditioners, processors, and digital wireless radio frequency (RF) links continue to become smaller, consume less power, and include higher levels of integration. The combination of these elements can provide sensing, acquisition, storage, and reporting functions in very small packages. Such sensing devices have been linked in wireless networks as described in the '127, patent and in the '9224, '194, '481, '541, '731, '637, '066, and '436 applications.
Networks of intelligent sensors have been described in a paper, “Intelligent Sensor Nodes Enable a New Generation of Machinery Diagnostics and Prognostics, New Frontiers in Integrated Diagnostics and Prognostics,” by F. M. Discenzo, K. A. Loparo, D. Chung, A. Twarowsk, 55th Meeting of the Society for Machinery Failure Prevention Technology, April, 2001, Virginia Beach.
Wireless sensors have the advantage of eliminating wiring installation expense and weight as well as connector reliability problems. However, wireless sensors still require power in order to operate. In some cases, sensors may be hardwired to a vehicle's power system. The wiring required for power defeats the advantages of wireless sensors and may be unacceptable for many applications. In addition, if a power outage occurs, critical data may be lost, at least during the time of the power outage.
Most prior wireless structural monitoring systems have relied on continuous power supplied by batteries. For example, a paper “An Advanced Strain Level Counter for Monitoring Aircraft Fatigue,” by Weiss, Instrument Society of America, ASI 72212, 1972, pages 105-108, 1972, described a battery powered inductive strain measurement system which measured and counted strain levels for aircraft fatigue. The disadvantage of traditional batteries, however, is that they become depleted and must be periodically replaced or recharged. This additional maintenance task adds cost and limits use to accessible locations.
Given the limitations of battery power, there has been a need for systems which can operate effectively using alternative power sources. Energy harvesting from vibrating machinery and rotating structures to provide power for such sensing devices and for wireless networks of sensors and/or actuators has been described in the commonly assigned '693 patent and in the '976, '679, '632, '642, and '731 applications.
A paper, “Energy Scavenging for wireless Sensor Networks with Special Focus on Vibrations,” by S. Roundy et al., Kluwer Academic Press, 2004, and a paper “Energy Scavenging for Mobile and Wireless Electronics,” Pervasive Computing, by J. A. Paradiso & T. Starner, IEEE CS and IEEE ComSoc, Vol 1536-1268, pp 18-26, 2005, describe various strategies for harvesting or scavenging energy from the environment. These sensing systems can operate truly autonomously because they do not require traditional battery maintenance
However, these energy harvesting systems have not been optimized for use on structures, such as aircraft and for use in certain networks. Thus, an improved system for monitoring is needed that harvests sufficient energy for operation and that can provide data over a network, and this solution is provided by this patent application.